The graft versus leukemia (GVL) effect is one of the most potent demonstrations of effective adoptive immunotherapy after allogeneic bone marrow transplantation (BMT). Unfortunately, GVL reactivity is often coexpressed with graft versus host disease (GVHD) which is the major complication of this therapy. For the past five years, we have examined a clinically feasible approach whereby thymidine kinase gene-modified alloreactive donor T cells are selectively eliminated early post-BMT to modulate donor T cell survival. Using this strategy in a murine BMT model, we have been able to show that there is a therapeutic window during which GVHD can be significantly reduced without loss of the GVL effect. We have also observed that GVHD results in a loss of self tolerance leading to the development of autoimmunity and that this is attributable to the absence of CD4+ CD25+ regulatory T cells (Tregs) post-BMT. These studies lead us to hypothesize that the interaction between alloreactive donor T cells and Tregs plays a dominant role in the biology of GVL and GVH responses and efforts to separate them in allogeneic marrow transplant recipients. To that end, the goal of this competing renewal is to examine how the temporal kinetics and tissue migration of alloreactive donor T cells and regulatory T cells during the evolution of GVL and GVH responses affect the ability to dissociate GVL/GVH reactivity. The specfic aims of the grant are: (1) to define the temporal kinetics of GVL and GVH reactivity, (2) to characterize the role of regulatory T cells in modulating the severity of GVHD, and (3) to determine how the interaction between Tregs and alloreactive donor T cells affects the ability to separate GVL and GVH reactivity. These studies will take advantage of novel, clinically relevant murine models to address these questions. The overall goal is that this proposal will provide important preclinical data that will further our understanding of how Tregs modulate GVL and GVH reactivity and will serve as a foundation for the application of this overall strategy into the clinic. The relevance of this project to public health derives from the fact that GVHD is the major complication of allogeneic stem cell transplantation. Better understanding of how to reduce this complication while at the same time preserving the beneficial effects that result from the transplant will lead to new therapies and better outcomes for patients.